fix typo in HeartMonitor.period helpstring

IPython/parallel/controller/heartmonitor.py

@@ -61,7 +61,7 b' class HeartMonitor(LoggingConfigurable):'
 
     period=CFloat(1000, config=True,
         help='The frequency at which the Hub pings the engines for heartbeats '
-        ' (in ms) [default: 100]',
+        '(in ms)',
     )
 
     pingstream=Instance('zmq.eventloop.zmqstream.ZMQStream')